int main (void)
{
    unsigned int MAX = 32;
    unsigned int index = 31;

    index = 31;
    index = (index + 1) % MAX;  // 這個容易理解
    printf ("index = %d/n", index);

    index = 31;
    index = (index + 1) & (MAX - 1);  // 這個運算效率高 
    printf ("index = %d/n", index);

    return;    
}